Catalyst composition

ABSTRACT

There is provided a catalyst carrier comprising a refractory inorganic material having a sodium solubilization rate no greater than 5 ppmw/5 minutes. There is further a catalyst comprising a refractory inorganic material carrier having a sodium solubilization rate no greater than 5 ppmw/5 minutes; and one or more catalytically reactive metals deposited on said carrier. There is also provided a catalyst suitable for the vapor phase production of alkylene oxide from olefins and oxygen comprising an alumina-based carrier having a sodium solubilization rate no greater than 5 ppmw/5 minutes; and catalytically reactive silver deposited on said carrier.

This is a continuation of application Ser. No. 09/392,521 filed Sep. 9,1999, now abandoned, the entire disclosure of which is herebyincorporated by reference. This application claims the benefit of U.S.Provisional Application No. 60/100,196 filed Sep. 14, 1998, the entiredisclosure of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a catalyst with improved catalytic properties,particularly a catalyst suitable for the preparation of epoxides.

BACKGROUND OF THE INVENTION

Methods have been described for lowering the total concentration ofsoluble species in the bulk of a catalyst carrier. These methodsgenerally involve a process by which the carrier is manufactured in sucha way so as to lower the concentration of those species throughout thebulk of the carrier. These approaches limit the formulation of carriers,often times with undesirable consequences such as high carrier density.

U.S. Pat. No. 4,797,270 discloses water washing to reduce the sodiumcontent of an alumina powder. The pH of the wash water may need to beadjusted for extraction of other metals and Japanese patent JP56164013discloses the use of a low pH (acid) to extract uranium and thorium froma calcined α-alumina raw material.

U.S. Pat. Nos. 4,361,504 and 4,366,092 suggest that ethylene oxidecatalyst be water washed after the deposition of silver or silver/goldon the carrier. EP-211521 discloses washing of a catalyst with hot waterto remove basic materials left on the catalyst from a silverimpregnation process or the physical deposition of alkali metals. U.S.Pat. No. 4,367,167 discloses a process for a supported catalyst whereinan impregnated support is immersed in an inert water immiscible organicsolvent containing a dissolved aliphatic amine. U.S. Pat. No. 4,810,689discloses depositing a silver compound, decomposing the silver compoundto silver in the presence of an alkali metal compound, removing organicdeposits by washing and introducing fresh alkali metal by impregnationduring or after the washing stage. U.S. Pat. Nos. 4,186,106 and4,125,480 disclose washing with an inert liquid after deposition of thecatalytic metal and before deposition of a promoter material.

The prior art remains concerned with the total amount of impurities;i.e., impurities throughout the bulk. Unfortunately, the impurityremoval techniques taught typically attack the carrier itself. It hassurprisingly been found that controlling the solubilization rate ofcertain species found on a carrier surface results in a catalyst withimproved catalytic properties.

SUMMARY OF THE INVENTION

According to the invention, there is provided a catalyst carriercomprising a material having a sodium solubilization rate no greaterthan 5 ppmw/5 minutes.

Another embodiment of the invention provides a catalyst comprising acarrier having a sodium solubilization rate no greater than 5 ppmw/5minutes; and one or more catalytically reactive metals deposited on saidcarrier.

A further embodiment of the invention provides a catalyst suitable forthe vapor phase production of epoxides comprising a carrier having asodium solubilization rate no greater than 5 ppmw/5 minutes; and one ormore catalytically reactive metals deposited on said carrier.

A further embodiment of the invention provides a catalyst suitable forthe vapor phase production of oxiranes from olefin and oxygen comprisinga carrier having a sodium solubilization rate no greater than 5 ppmw/5minutes; and catalytically reactive silver deposited on said carrier.

DETAILED DESCRIPTION

It has been found that carriers which have a controlled solubilizationrate, in particular controlled sodium and/or soluble silicatesolubilization rates, provide catalysts with improved catalyticproperties, such as activity, selectivity and activity and/orselectivity performance over time. Controlling the solubilization rateis believed to work to improve the properties of most catalysts, nomatter how impure the bulk carrier material. Further, controlling thesolubilization rate will work for organic or inorganic carriers.

The typical carrier of the invention has a sodium solubilization rate inboiling water which is controlled to be no greater than 5 ppmw/5minutes. As used herein, boiling water is deemed to have a temperatureof 100° C. “Solubilization rate” as used herein refers to the measurablesolubilization rate of the sodium in a solution after the carrier isplaced in the solution for a specified time and at a ratio of boilingsolution to carrier of 3:1. Thus, a solubilization rate in boiling waterof 5 ppmw sodium/5 minutes is the amount of sodium measured in the waterafter the carrier has been in the boiling water for five minutes.

Carriers are commonly inorganic materials such as, for example,alumina-, silica-, or titania-based compounds, or combinations thereof,such as alumina-silica carriers. Carriers may also be made fromcarbon-based materials such as, for example, charcoal, activated carbon,or fullerenes. Ionizable species typically present on the inorganic typecarriers include sodium, potassium, aluminates, soluble silicate,calcium, magnesium, aluminosilicate, cesium, lithium, and combinationsthereof. Of particular concern are the ionizable anionic species presenton the surface, particularly ionizable silicates. The solubilizationrate of silicates may be measured by inductively coupled plasma (ICP)techniques and the amount of silicon species on a surface may bemeasured by x-ray photoelectron spectroscopy (XPS); however, sincesodium is soluble in the same solutions that silicates are soluble in,the solubilization rate of sodium becomes a simpler check of the ionicspecies removal and it has been chosen as the indicator to define thepresent invention. Another measurement technique is to measure theelectrical conductivity of the treatment solution.

Control of the solubilization rate may be obtained by a multiple ofmeans. The raw materials for the carrier can be tightly controlled, forexample. Or the surface of the carrier may be treated. As used herein,the “surface” of the carrier is that area of the carrier which may bemeasured by BET analysis. Specifically, the surface of the carrier isthe site at which reaction takes place. Lowering the concentration ofionizable species on the surface of the carrier has been found to be aneffective and cost efficient means of achieving the desired sodiumsolubilization rate. An “ionizable” species is a species which iscapable of being rendered ionic, where the term “ionic” or “ion” refersto an electrically charged chemical moiety.

Lowering the surface solubilization rate of ionizable species may beaccomplished by any means (i) which is effective in rendering theionizable species ionic and removing the species, or (ii) which rendersthe ionizable species insoluble, or (iii) which renders the ionizablespecies immobile; however, use of aggressive medias is discouraged asthese medias tend to dissolve the carrier, extract too much materialfrom the bulk, and generate acidic or basic sites in the pores. Acids,which are considered aggressive media, will remove the cations on acarrier but are fairly ineffectual in removing the undesirable anions,such as silicates. Effective means of lowering concentration includewashing the carrier; ion exchange; volatilizing, precipitating, orsequestering the impurities; causing a reaction to make the ionizablespecies on the surface insoluble; and combinations thereof. The bulkcarrier may be treated, or the raw materials used to form the carriermay be treated before the carrier is manufactured. Even greaterimprovements in solubilization rate control are seen when both thecarrier raw materials and the finished carrier are treated.

To make a catalyst from the carrier, the carrier is typicallyimpregnated with metal compound(s), complex(es) and/or salt(s) dissolvedin a suitable solvent sufficient to deposit or impregnate acatalytically effective amount of metal on the carrier. As used herein,“catalytically effective amount” means an amount of metal that providesa measurable catalytic effect. For example, a catalytically effectiveamount of metal when referring to an olefin epoxidation catalyst is thatamount of metal which provides a measurable conversion of olefin andoxygen to alkylene oxide. In addition, one or more promoters may also bedeposited on the carrier either prior to, coincidentally with, orsubsequent to the deposition of the catalytically reactive metal. Theterm “promoter” as used herein refers to a component which workseffectively to provide an improvement in one or more of the catalyticproperties of the catalyst when compared to a catalyst not containingsuch component.

Further improvement in the catalyst properties are seen when the metaldeposition is effected by contacting the carrier with an impregnationsolution whose hydrogen ion activity has been lowered. “Hydrogen ionactivity” as used herein is the hydrogen ion activity as measured by thepotential of a hydrogen ion selective electrode. As used herein, asolution with “lowered” hydrogen ion activity refers to a solution whosehydrogen activity has been altered by the addition of a base, such thatthe hydrogen ion activity of the altered solution is lowered compared tothe hydrogen ion activity of the same solution in an unaltered state.The base selected to alter the solution may be chosen from any base orcompound with a pKb lower than the original impregnation solution. It isparticularly desirable to chose a base which does not alter theformulation of the impregnation solution; i.e., which does not alter thedesired metals concentration in the impregnation solution and depositedon the carrier. Organic bases will not alter the impregnation solutionmetals concentrations, examples of which are tetraalkylammoniumhydroxides and 1,8-bis-(dimethylamino)-naphthalene. If changing themetals concentration of the impregnation solution is not a concern,metal hydroxides may be used.

When the impregnation solution is at least partially aqueous, anindication of the change in the hydrogen activity may be measured with apH meter, with the understanding that the measurement obtained is not pHby a true, aqueous definition. ‘“Measured pH”’ as used herein shall meansuch a non-aqueous system pH measurement using a standard pH probe. Evensmall changes in the “measured pH” from the initial impregnationsolution to that with added base are effective and improvements incatalytic properties continue as the “measured pH” change increases withbase addition. High base additions do not seem to adversely affectcatalyst performance; however, high additions of hydroxides have beenseen to cause sludging of the impregnation solution, creatingmanufacturing difficulties. When the base addition is too low, thehydrogen ion activity will not be affected. The hydrogen ion activitylowering procedure is also quite effective when used by itself; i.e.,when no ionizable species concentrations are lowered prior toimpregnation.

The impregnated carrier, also known as a catalyst precursor, is dried inthe presence of an atmosphere which also reduces the catalytic metal.Drying methods known in the art include steam drying, drying in anatmosphere with a controlled oxygen concentration, drying in a reducingatmosphere, air drying, and staged drying using a suitable ramped orstaged temperature curve.

By way of example, the invention will be described in more detail for acatalyst suitable for the vapor phase production of epoxides, also knownas an epoxidation catalyst.

An epoxidation catalyst typically comprises an inorganic carrier, suchas for example, and alumina-based carrier such as α-alumina, with one ormore catalytically reactive metals deposited on the carrier. The carriertypically contains certain ionizable species, for example an α-aluminacarrier, typically contains species including sodium, potassium,aluminates, soluble silicates, calcium, magnesium, aluminosilicates, andcombinations thereof. It has been found that silicates, and certainother anions, are particularly undesirable ionizable species in anepoxidation catalyst. As already described, the solubilization rate ofsilicons/silicates may be measured by ICP and by XPS; however, sincesodium is soluble in the same solutions that silicates are soluble in,the solubilization rate of sodium becomes a simpler check of the ionicspecies removal. Another measurement technique is to measure theelectrical conductivity of the treatment solution.

According to the invention, the sodium solubilization rate of thecarrier is controlled. The solubilization rate may be controlled bylowering the concentration of ionizable species on the surface.Ionizable species concentration may be lowered by means which render theionizable species ionic and thereafter removing the ionic species, or byrendering those ionizable species insoluble, or rendering the ionizablespecies immobile. For example, the carrier, or the raw materials of thecarrier, may be subjected to washing; ion exchange; volatilizing,precipitating, or sequestering the impurities; causing a reaction tomake the ionizable species on the surface insoluble; and combinationsthereof. When washing is used, the sodium solubilization rate in 3:1 w/wboiling water is preferably controlled to less than 5 ppmw Na/5 minutes.

The carrier having the controlled solubilization rate is impregnatedwith metal ions or compound(s), complex(es) and/or salt(s) dissolved ina suitable solvent sufficient to cause the desired deposition on thecarrier. When silver is the deposition material, a typical deposition isfrom about 1 to about 40 percent by weight, preferably from about 1 toabout 30 percent by weight silver, basis the weight of the totalcatalyst. The impregnated carrier is subsequently separated from thesolution and the deposited metal(s) compound is reduced to metallicsilver.

One or more promoters may be deposited either prior to, coincidentallywith, or subsequent to the deposition of the metal. Promoters forepoxidation catalysts are typically selected from sulfur, phosphorus,boron, fluorine, Group IA through Group VIII metals, rare earth metals,and combinations thereof. The promoter material is typically compound(s)and/or salt(s) of the promoter dissolved in a suitable solvent.

For olefin epoxidation oxide catalysts, Group IA metals are typicallyselected from potassium, rubidium, cesium, lithium, sodium, andcombinations thereof; with potassium and/or cesium and/or rubidium beingpreferred. Even more preferred is a combination of cesium plus at leastone additional Group IA metal, such as cesium plus potassium, cesiumplus rubidium, or cesium plus lithium. Group IIA metals are typicallyselected from magnesium, calcium, strontium, barium, and combinationsthereof, Group VIII transition metals are typically selected fromcobalt, iron, nickel, ruthenium, rhodium, palladium, and combinationsthereof; and rare earth metals are typically selected from lanthanum,cerium, neodymium, samarium, gadolinium, dysprosium, erbium, ytterbium,and mixtures thereof. Non-limiting examples of other promoters includeperrhenate, sulfate, molybdate, tungstate, chromate, phosphate, borate,sulfate anion, fluoride anoin, oxyanions of Group IIIB to VIB, oxyanionsof an element selected from Groups III through VIIB, alkali(ne) metalsalts with anions of halides, and oxyanions selected from Groups IIIA toVIIA and IIIB through VIIB. The amount of Group IA metal promoter istypically in the range of from about 10 ppm to about 1500 ppm, expressedas the metal, by weight of the total catalyst, and the Group VIIb metalis less than about 3600 ppm, expressed as the metal, by weight of thetotal catalyst.

For further improvement in catalytic properties, the hydrogen ionactivity of the impregnation solution is optionally lowered, such as bythe addition of a base. The typical impregnation solution for anepoxidation catalyst begins quite basic, so a strong base is used tofurther lower the hydrogen ion activity. Examples of strong basesinclude alkyl ammonium hydroxide such as tetraethylammonium hydroxide,and metal hydroxide such as lithium hydroxide and cesium hydroxide. Inorder to maintain the desired impregnation solution formulation andmetal loading, an organic base such as tetraethylammonium hydroxide ispreferred. Base additions in these systems typically result in a“measured pH” change ranging up to about 3 pH units, realizing that the“measured pH” is not a true pH since the impregnation system is notaqueous.

The carrier employed in these catalysts in its broadest aspects can beany of the large number of conventional, porous refractory catalystcarriers or carrier materials which are considered relatively inert.Such conventional materials are known to those skilled in the art andmay be of natural or synthetic origin. Carriers for epoxidationcatalysts are preferably of a macroporous structure and have a surfacearea below about 10 m²/g and preferably below about 3 m²/g. Examples ofcarriers for different catalysts are the aluminum oxides (including thematerials sold under the trade name “Alundum”), charcoal, pumice,magnesia, zirconia, kieselguhr, fuller's earth, silicon carbide, porousagglomerates comprising silica and/or silicon carbide, silica, magnesia,selected clays, artificial and natural zeolites, alkaline earthcarbonates, and ceramics. Refractory carriers especially useful in thepreparation of olefin epoxidation catalysts comprise the aluminousmaterials, in particular those comprising α-alumina. In the case ofα-alumina-containing carriers, preference is given to those having aspecific surface area as measured by the B.E.T. method of from about0.03 m²/g to about 10 m²/g, preferably from about 0.05 m²/g to about 5m²/g, more preferably from about 0.1 m²/g to about 3 m²/g, and a waterpore volume as measured by conventional water absorption techniques offrom about 0.1 to about 0.75 cc/g by volume. The B.E.T. method fordetermining specific surface area is described in detail in Brunauer,S., Emmett, P. Y. and Teller, E., J. Am. Chem. Soc., 60, 309–16 (1938).

Certain types of α-alumina containing carriers are particularlypreferred. These α-alumina carriers have relatively uniform porediameters and are more fully characterized by having B.E.T. specificsurface areas of from about 0.1 m²/g to about 3 m²/g, preferably fromabout 0.1 m²/g to about 2 m²/g, and water pore volumes of from about0.10 cc/g to about 0.55 cc/g. Manufacturers of such carriers includeNorton Chemical Process Products Corporation and United Catalysts, Inc.(UCI).

The resulting epoxidation catalysts just described are used for thevapor phase production of epoxides. A typical epoxidation processinvolves loading catalysts into a reactor. The feedstock to beconverted, typically a mixture of ethylene, oxygen, carbon dioxide,nitrogen and ethyl chloride, is passed over the catalyst bed at pressureand temperature. The catalyst converts the feedstock to an outlet streamproduct which contains ethylene oxide. Nitrogen oxides (NO_(x)) may alsobe added to the feedstock to boost catalyst conversion performance.

Having generally described the invention, a further understanding may beobtained by reference to the following examples, which are provided forpurposes of illustration only and are not intended to be limiting unlessotherwise specified.

EXAMPLES

Carriers

The properties of the carriers used in the Examples are given in TableI.

TABLE I Carrier A B C D B.E.T. Surface Area (m²/g)^((a)) 0.84 0.97 0.780.87 Water Absorption (%) 39.7 46.2 37.6 43.4 Crush Strength (kg)^((b))6.53 8.07 12.29 5.44 Total Pore Volume (cc/g)^((c)) 0.408 0.460 0.390Median Pore Diameter (microns)^((c)) 1.8 2.7 1.3 SiO₂ (% w) 0.5 0.8 0.10.5 Bulk Acid-Leachable Na (ppmw) 438 752 186 339 Bulk Acid-Leachable K(ppmw) 85 438 109 37 Bulk Acid-Leachable Ca (ppmw) 207 508 526 123 BulkAcid-Leachable Al (ppmw) 744 1553 657 499 Bulk Acid-Leachable SiO₂(ppmw) 808 1879 1560 600 alpha-Alumina (% w) Bal Bal Bal Bal ^(a)Methodof Brunauer, Emmett and Teller, loc. cit. ^(b)Flat Plate Crush Strength,single pellet. ^(c)Determined by mercury intrusion to 3.8 × 10⁸ Pa usingMicromeritics Autopore 9200 or 9210 (130° contact angle, 0.473 N/msurface tension of Hg).Carrier Water Washing Procedures for Examples 1, 2, 3, 4, 6, 7, 10, 12

Carrier washing was carried out by immersing 100 grams of carrier in 300grams of boiling de-ionized water for 15 minutes. The carrier was thenremoved and placed in a fresh 300 grams of boiling water for another 15minutes. This procedure was repeated once more for a total of threeimmersions, at which point the carrier was separated from the water anddried in a well ventilated oven at 150° C. for 18 hours. The driedcarrier was then used for preparation of a catalyst by the proceduresoutlined in the following Examples.

Impregnation Solution

A silver-amine-oxalate stock solution was prepared by the followingprocedure:

415 g of reagent-grade sodium hydroxide were dissolved in 2340 mlde-ionized water and the temperature was adjusted to 50° C.

1699 g high purity “Spectropure” silver nitrate were dissolved in 2100ml de-ionized water and the temperature was adjusted to 50° C.

The sodium hydroxide solution was added slowly to the silver nitratesolution, with stirring, while maintaining a solution temperature of 50°C. The mixture was stirred for 15 minutes, then the temperature waslowered to 40° C.

Water was removed from the precipitate created in the mixing step andthe conductivity of the water, which contained sodium and nitrate ions,was measured. An amount of fresh deionized water equal to the amountremoved was added back to the silver solution. The solution was stirredfor 15 minutes at 40° C. The process was repeated until the conductivityof the water removed was less than 90 μmho/cm. 1500 ml fresh deionizedwater was then added. 630 g of high-purity oxalic acid dihydrate wereadded in approximately 100 g increments. The temperature was keep at 40°C. and the pH was kept above 7.8.

Water was removed from the mixture to leave a highly concentratedsilver-containing slurry. The silver oxalate slurry was cooled to 30° C.

699 g of 92%w ethylenediamine (8% de-ionized water) was added whilemaintaining a temperature no greater than 30° C. The resulting solutioncontained approximately 27–33%w silver.

Enough 45%w aqueous CsOH and water was added to this solution to give afinished catalyst having 14.5%w silver and a desired cesium loading (seeExamples).

Sodium Measurement Procedures

The sodium solubilization rate of selected carriers was determined bymeasuring the sodium content of the extracting medium with an Orionmodel no. 8611BN sodium selective electrode connected to an Orion model290A voltmeter, and by XPS. The silicate solubilization rates weremeasured by XPS. In a typical experiment, 300 grams of carrier wasboiled in 900 grams of de-ionized water for a total of fifteen minutes.During this period, 3 ml aliquots were taken at predetermined intervals.The sodium content of each aliquot was analyzed at 25° C. usingprocedures well established for ion selective electrodes. The sodiumconcentration in the solution sampled at 5 minutes is used to evaluatethe carrier as being a good or poor candidate for catalyst preparation.Results are given in Table II.

TABLE II Sodium Solubilization Rates for Selected α-Alumina CarriersBulk Na Extracted Na Extracted Na Unwashed Carrier Unwashed CarrierWashed Carrier Carrier (ppmw)^(a) (ppmw) (ppmw) A 438 9.2 1.3 A^(b) 4389.2 1.2 B 752 9.2 1.8 C 186 10.2  — ^(a)From Table I. ^(b)Followingammonium acetate exchange as described in Example 8.

pH Measurement Procedures

Silver solution pH measurements were done using a Metrohm model 744 pHmeter, employing a model 6.0220.100 combination electrode and a Pt 100model 6.1110.100 resistance thermometer for temperature compensation.The meter was calibrated with commercially available buffer solutionsbefore each use. In a typical measurement, a 50 cc aliquot of the dopedsilver solution to be used for a catalyst impregnation was filtered intoa 100 cc glass beaker through a 2 micron filter attached in-line to aplastic syringe. The pH probe was lowered into the magnetically stirredsolution, and the reading obtained after 3 minutes was recorded as theequilibrated pH. The probe was cleaned between each measurement withdeionized water, and checked for calibration. Special care was taken toprevent accumulation of AgCl solids on the electrode membrane. Suchaccumulation was removed by soaking the probe in ammonium hydroxidesolution, as recommended by the manufacturer.

Example 1

A catalyst pre-cursor was prepared from Carrier A by first subjectingthe carrier to carrier washing. Following the wash, approximately 30grams of washed Carrier A were placed under a 25 mm Hg vacuum for 1minute at ambient temperature. Approximately 50 grams of theimpregnating solution was then introduced to submerse the carrier, andthe vacuum was maintained at 25 mm Hg for an additional 3 minutes. Thecesium target was 450 ppmw/gram finished catalyst. The vacuum was thenreleased and the excess impregnating solution was removed from thecatalyst pre-cursor by centrifugation at 500 rpm for two minutes. Thecatalyst pre-cursor was then dried while being shaken at 240° C. for 4minutes in a stream of air flowing at 11.3 m³/hr.

Example 1a (Comparative)

Carrier A was impregnated as described in Example 1; however, thecarrier was not subjected to carrier washing. The cesium target was 400ppmw/gram finished catalyst.

Example 2

Carrier B was subjected to carrier washing and impregnation as describedin Example 1. The cesium target was 450 ppmw/gram finished catalyst.

Example 2a (Comparative)

Carrier B was impregnated as described in Example 1; however, thecarrier was not subjected to carrier washing. The cesium target was 400ppmw/gram finished catalyst.

Example 3

Carrier C was subjected to carrier washing and impregnation as describedin Example 1. The cesium target was 300 ppmw/gram finished catalyst.

Example 3a (Comparative)

Carrier C was impregnated as described in Example 1; however, thecarrier was not subjected to carrier washing. The cesium target was 360ppmw/gram finished catalyst.

Example 4

Carrier A was subjected to carrier washing and impregnation as describedin Example 1. The cesium target was 450 ppmw/gram finished catalyst. Inaddition, 35% aqueous tetraethylammonium hydroxide (TEAH) was added tothe stock impregnation solution at a target of 117.8 micromoles OH⁻/ccAg solution, to lower the hydrogen ion activity to a “measured pH” of13.2.

Example 5

100 g of Carrier A were immersed in 300 ml of boiling 5%w TEAH for 15min, then immersed six times in 300 ml of boiling de-ionized water for15 minutes each. The carrier was then removed and dried in a wellventilated oven at 150° C. for 18 hours. The carrier was thenimpregnated with a cesium target of 400 ppmw/gram finished catalyst. Inaddition, 35%w TEAH was added to the stock impregnation solution at atarget of 117.8 micromoles OH⁻/cc Ag, to lower the hydrogen ion activityto a “measured pH” of 13.6.

Example 6

Carrier A was subjected to carrier washing and impregnation as describedin Example 1. The cesium target was 720 ppmw/gram finished catalyst. Inaddition, TEAH was dissolved in water and added to the stock solution ata target of 117.8 micromoles OH⁻/cc Ag, to lower the hydrogen activityto a “measured pH” of 13.2, and NH₄ReO₄ was dissolved in water and addedto the stock solution to provide 1.5 micromoles Re/gram finishedcatalyst.

Example 7

Carrier A was subjected to carrier washing and impregnation as describedin Example 1. The cesium target was 450 ppmw/gram finished catalyst. Inaddition, LiOH was dissolved in water and added to the stockimpregnation solution to lower the hydrogen ion activity to a “measuredpH” of 13.2.

Example 7a (Comparative)

Carrier A was impregnated as described in Example 7; however, thecarrier was not subjected to carrier washing. The cesium target was 400ppmw/gram finished catalyst.

Example 8

300 g of Carrier A were immersed in 900 ml of a boiling 0.1 M solutionof ammonium acetate for 15 min, then immersed in 300 ml of de-ionizedwater at 25° C. for 15 minutes, followed by immersion three times in 300ml of boiling deionized water for 15 minutes each. The carrier was thenremoved and dried in a well ventilated oven at 150° C. for 18 hours. Thecarrier was then impregnated as described in Example 1. The cesiumtarget was 450 ppmw/gram finished catalyst. In addition, LiOH wasdissolved in water and added to the stock impregnation solution to lowerthe hydrogen ion activity to a “measured pH” of 13.2.

Example 9

The α-alumina source material for Carrier A was washed with de-ionizedwater at 25° C., then homogenized with the same ingredients used to formCarrier A before extruding, drying, and firing in a muffle furnace. Theresulting carrier was designated Carrier D. Carrier D was used toprepare a catalyst in the same manner as described in Example 1. Thecesium target was 510 ppmw/gram finished catalyst. In addition, LiOH wasdissolved in water and added to the stock impregnation solution to lowerthe hydrogen ion activity to a “measured pH” of 13.2.

Example 9a (Comparative)

A catalyst was prepared from Carrier D in the same manner as outlined inExample 9; however, the carrier was not subjected to carrier washing.The cesium target was 360 ppmw/gram finished catalyst.

Example 10

100 g of Carrier A were immersed in 300 ml of a boiling 0.1 M solutionof barium acetate at 25° C. for 15 min, then immersed in 300 ml ofde-ionized water at 25° C. for 15 minutes, followed by immersion threetimes in 300 ml of boiling de-ionized water for 15 minutes each. Thecarrier was then removed and dried in a well ventilated oven at 150° C.for 18 hours. The carrier was then impregnated as described inExample 1. The cesium target was 400 ppmw/gram finished catalyst. Inaddition, LiOH was dissolved in water and added to the stockimpregnation solution to lower the hydrogen ion activity to a “measuredpH” of 13.2.

Example 11

Carrier A was subjected to carrier washing and impregnation as describedin Example 1. The cesium target was 650 ppmw/gram finished catalyst. Inaddition, LiOH was dissolved in water and added to the stockimpregnation solution to lower the hydrogen ion activity to a “measuredpH” of 13.2 and NH₄ReO₄ was dissolved in water and added to the stockimpregnation solution to provide 1.5 micromoles Re/gram finishedcatalyst.

The catalysts of Examples 1–11 were used to produce ethylene oxide fromethylene and oxygen. 3 to 5 grams of crushed catalyst were loaded into a6.35 mm inside diameter stainless steel U-shaped tube. The U tube wasimmersed in a molten metal bath (heat medium) and the ends wereconnected to a gas flow system. The weight of the catalyst used and theinlet gas flow rate were adjusted to achieve a gas hourly space velocityof 6800 cc of gas per cc of catalyst per hour. The inlet gas pressurewas 1450 kPa.

The gas mixture passed through the catalyst bed (in a once-throughoperation) during the entire test run (including start-up) consisted of25% ethylene, 7.0% oxygen, 5% carbon dioxide, 63% nitrogen, and 2.0 to6.0 ppmv ethyl chloride.

The initial reactor (heat medium) temperature was 180° C. Thetemperature was ramped at a rate of 10° C. per hour from 180° C. to 225°C., and then adjusted so as to achieve a constant ethylene oxide levelof 1.5 %v in the outlet gas stream. Performance data at this conversionlevel are usually obtained when the catalyst has been on stream for atotal of at least 1–2 days. Due to slight differences in feed gascomposition, gas flow rates, and the calibration of analyticalinstruments used to determine the feed and product gas compositions, themeasured selectivity and activity of a given catalyst may vary slightlyfrom one test run to the next.

The initial performance values for selectivity at 1.5% ethylene oxidewere measured and are reported in Table III.

TABLE III Performance Characteristics of Catalysts Prepared FromUnwashed and Washed α-Alumina Pre-Impregnation Base ImpregnatingSolution Selectivity Temperature Example Carrier Condition Addition“measured pH” (%) (° C.)  1 A water wash none 11.2 82.7 229  1a A nowash none 11.2 81.3 237  2 B water wash none 11.2 82.5 226  2a B no washnone 11.2 82.0 232  3 C water wash none 11.2 82.0 229  3a C no wash none11.2 82.0 235  4 A water wash TEAH 13.2 82.7 226  5 A TEAH wash + TEAH13.6 82.7 222 water wash  6 A water wash TEAH 13.2 89.4 245  7 A waterwash LiOH 13.2 82.7 226  7a A no wash LiOH 13.2 82.0 227  8 A ammoniumacetate wash LiOH 13.2 83.1 222  9 D raw material wash + LiOH 13.2 82.7222 carrier body wash  9a D raw material wash + LiOH 13.2 83.0 225 nocarrier body wash 10 A barium acetate wash LiOH 13.2 82.7 226 11 A waterwash LiOH 13.2 86.2 232

It can be seen that significant improvement in catalyst properties areseen when the sodium solubilization rate is lowered. Carriers A and Bhave dramatically lower sodium solubilization rates (see Table II) afterbeing subjected to the Carrier Washing Procedure. Notice that despitethe lower bulk sodium for Carrier C, it has a high sodium solubilizationrate. Even further improvement is seen when the material used to makethe carrier is washed before the carrier is formed, Carrier D.

The hydrogen ion activity of the deposition solution for catalysts inExamples 4–11 was lowered by the addition of a base. It can be seen thatlowering the hydrogen ion activity of the deposition solution furtherimproves the catalytic properties. It is also evident that thephenomenon of the pH effect is not restricted to a particular catalystformulation, as best illustrated in Examples 6 and 11, where aselectivity enhancing dopant, such as rhenium, is added to theimpregnating solution.

It will be apparent to one of ordinary skill in the art that manychanges and modifications may be made to the invention without departingfrom its spirit or scope as set forth herein.

1. A process for preparing a catalyst, said process comprising:selecting a carrier having a sodium solubilization rate no greater than5 ppmw/5 minutes; depositing one or more catalytically reactive metalscomprising silver on said carrier; and depositing one or more promotersselected from phosphorus, boron, fluorine, lithium, sodium, rubidium,Group IIA through Group VIII metals, rare earth metals, and combinationsthereof prior to, coincidentally with, or subsequent to the depositionof said one or more catalytically reactive metals.
 2. A processaccording to claim 1 wherein the one or more promoters comprise lithium.3. A process according to claim 2 wherein the one or more promoterscomprise in addition a Group IA metal which is selected from potassium,rubidium, cesium, sodium, and combinations thereof.
 4. A processaccording to claim 1 wherein the one or more promoters are selected fromphosphorus, boron, fluorine, Group IIA through Group VIII metals, rareearth metals, and combinations thereof.
 5. A process according to claim4 wherein said Group IIA metal is selected from magnesium, calcium,strontium, barium, and combinations thereof.
 6. A process according toclaim 4 wherein said Group VIII metal is selected from cobalt, iron,nickel, ruthenium, rhodium, palladium, and combinations thereof.
 7. Aprocess according to claim 4 wherein said rare earth metal is selectedfrom lanthanum, cerium, neodymium, samarium, gadolinium, dysprosium,erbium, ytterbium, and combinations thereof.
 8. A process according toclaim 4 wherein the one or more promoters comprise rhenium.
 9. A processaccording to claim 1 wherein said sodium solubilization rate has beenachieved by a means effective in rendering ionizable species on thecarrier surface ionic and removing that species, or rendering theionizable species insoluble, or rendering the ionizable speciesimmobile.
 10. A process according to claim 9 wherein said means isselected from washing, ion exchange, volatilizing, impurity control,precipitation, sequestration, and combinations thereof.
 11. A processaccording to claim 1 wherein said metal is deposited on said carrier bysubmersing said carrier in an impregnation solution wherein a hydrogenion activity of said solution is lowered.
 12. A process according toclaim 11 wherein said hydrogen ion activity is lowered by addition of abase to said impregnation solution.
 13. A process according to claim 1wherein said carrier is an alumina-based carrier.
 14. A processaccording to claim 13 wherein said alumina-based carrier is α-alumina.15. A process according to claim 1 wherein the amount of a Group IAmetal, if present, is in the range of from about 10 ppm to about 1500ppm, by weight of the total catalyst, expressed as the metal; the amountof a Group VIIB metal, if present, is less than about 3600 ppm, byweight of the total catalyst, expressed as the metal; and the amount ofsilver is in the range of from about 1 percent by weight to about 40percent by weight of the total catalyst.
 16. A process according toclaim 1 wherein said carrier has a surface area in the range of fromabout 0.05 m²/g to about 10 m²/g.
 17. A catalyst comprising a carrierhaving a sodium solubilization rate no greater than 5 ppmw/5 minutes;and deposited on said carrier a catalytically effective amount of one ormore catalytically reactive metals comprising silver, and one or morepromoters selected from phosphorus, boron, fluorine, lithium, sodium,rubidium, Group IIA through Group VIII metals, rare earth metals, andcombinations thereof.
 18. A catalyst according to claim 17 wherein theone or more promoters comprise lithium.
 19. A catalyst according toclaim 18 wherein the one or more promoters comprise in addition a GroupIA metal which is selected from potassium, rubidium, cesium, sodium, andcombinations thereof.
 20. A catalyst according to claim 18 wherein theone or more promoters comprise in addition a Group IA metal which iscesium.
 21. A catalyst according to claim 17 wherein the one or morepromoters are selected from phosphorus, boron, fluorine, Group IIAthrough Group VIII metals, rare earth metals, and combinations thereof.22. A catalyst according to claim 21 wherein said Group IIA metal isselected from magnesium, calcium, strontium, barium, and combinationsthereof.
 23. A catalyst according to claim 21 wherein said Group VIIImetal is selected from cobalt, iron, nickel, ruthenium, rhodium,palladium, and combinations thereof.
 24. A catalyst according to claim21 wherein said rare earth metal is selected from lanthanum, cerium,neodymium, samarium, gadolinium, dysprosium, erbium, ytterbium, andcombinations thereof.
 25. A catalyst according to claim 21 wherein theone or more promoters comprise rhenium.
 26. A catalyst according toclaim 17 wherein said carrier is an alumina-based carrier.
 27. Acatalyst according to claim 26 wherein said alumina-based carrier isα-alumina.
 28. A catalyst according to claim 17 wherein the amount of aGroup IA metal, if present, is in the range of from about 10 ppm toabout 1500 ppm, by weight of the total catalyst, expressed as the metal;the amount of a Group VIIB metal, if present, is less than about 3600ppm, by weight of the total catalyst, expressed as the metal; and theamount of silver is in the range of from about 1 percent by weight toabout 40 percent by weight of the total catalyst.
 29. A catalystaccording to claim 17 wherein said carrier has a surface area in therange of from about 0.05 m²/g to about 10 m²/g.